The present invention relates to a long-range artillery shell that is fin-stabilised in its trajectory and which is designed to be fired in a rifled gun barrel and thus has a so-called slipping driving band as main contact surface with the inside of the barrel and, after it has left the barrel, has deployable stabilisation fins. Special features of the shell as claimed in the present invention are the design of the stabilisation fins, the way they are deployed, and the fact that while retracted they are inside a propellant chamber or propellant motor of a base-bleed unit incorporated in the shell.
A possible reason for choosing a fin-stabilised artillery shell instead of a spin-stabilised shell is that one could want to make it guideable on its way to the target and it is much easier to correct the trajectory of a fin-stabilised shell than a spin-stabilised shell, and this applies irrespective of whether the correction to the trajectory is to be achieved by impulse motors, guidance fins or some other method.
A requirement for the shell as claimed in the present invention is that it shall be possible to provide it with extra long range. A method used increasingly in recent years to achieve extreme long ranges even with old tube-firing artillery is the base-bleed technique used to eliminate rear-end turbulence and the underpressure formed behind shells as they fly through the atmosphere, both of which have a decelerating effect that shortens range. The base-bleed technique involves the installing, in the rear section of the shell, of a combustion chamber filled with slow burning propellant which, while it burns, generates gases that flow out through an orifice in the rear face of the shell at a pre-determined rate, thus eliminating and equalising the decelerating turbulence and underpressure behind the shell.
However, when providing a shell with a base-bleed unit as well as stabilisation fins there arises a problem regarding the location of the latter as the base-bleed unit must be located in the rear of the shell with at least one gas outflow outlet in the rear face of the shell, while the fins also need to be located in the rear section of the shell as far as possible from the centre of gravity of the shell. An extra problem is that to enable the shell to be fired from a rifled barrel the fins must be fully retractable inside the minimum diameter of the barrel while not occupying too large a volume inside the shell thereby preventing the use of this space for the cargo that justifies the existence of the shell.
The present invention now offers a solution to the problem with retractable fins that involves an advantageous function while they are in retracted mode and which enables location of the fins very close to the rear face of the shell, i.e. at precisely the position where they need to be located.
As claimed in the present invention the fins are initially retracted radially or accommodated in the base-bleed unit""s propellant chamber or motor section via slots or through-openings in its outer wall. In retracted mode the fins are thus enclosed by radial protective walls that remain in place even after the fins have deployed. Naturally the protective walls and the spaces occupied by the fins occupy a small part of the total volume of the propellant chamber but, at the same time, one obtains what can be considered a division of the propellant chamber into a number of sectors separated from each other by the protective walls of the fin compartments while these sectors remain in mutual contact via a central axial space around the longitudinal axis of the propellant chamber that leads to the above mentioned gas outflow orifice. The present invention, namely, does not permit the fins and the protective walls surrounding them in retracted mode to extend all the way to the central axis of the propellant chamber; instead, they are terminated just before this point.
This arrangement, as claimed in the present invention, provides several advantages. Firstly the fins are optimally located, i.e. at the extreme rear of the shell, and secondly the location of the fins in retracted mode does not impact negatively on the active cargo of the shell, and thirdly the location of the fins involves only a slight extension of the propellant chamber of the base-bleed unit to achieve the same volume that was previously available for an active propellant cargo, and finally by subdividing the propellant chamber into sectors one obtains xe2x80x98freexe2x80x99 access to an efficient division and support of the base-bleed propellant. The latter aspect has shown itself to be at least as important, since previously there were major problems in producing a base-bleed unit suitable for slow-burning propellant elements of sufficient size and strength to withstand the accelerations involved in firing while also holding them together until their active burnout. Consequently, it was previously necessary to devise special propellant supports inside the combustion chamber of the base-bleed unit. An example of such a propellant support in the form of a support cupola initially arranged internally around the outlet nozzle of a base-bleed unit is described in our own Swedish patent number 461477.
To provide the fins with a greater length than is immediately enabled by the diameter of the shell the fins can be given a telescopic function, i.e. each fin is produced in the form of two or more initiallyxe2x80x94before deploymentxe2x80x94telescoped parts. To extend these fin elements, both from their compartments between the protective walls inside the propellant chamber and from each other, parts of the gas pressure that propels the shell from the barrel can be used in a way described in more detail below. This gas pressure can subsequently, to a greater or lesser extent, also be supplemented by the gas pressure generated inside the propellant chamber of the base-bleed unit when the propellant therein is ignited. The available gas pressure is thus used to push the fins through their respective slots in the side wall of the shell and to extend them from their telescoped mode. To provide a desirable seal when the fins have reached their fully deployed position their inner edges should preferably be designed so that they are slightly flared inwards towards the inside of the propellant chamber, so that as soon as they have each reached their fully deployed position they become wedged firmly in their respective slots in the outer wall of the propellant chamber or become wedged/locked at the extremity of each first fin element.
Of course, to provide extension of the telescoped fin elements various completely mechanical devices, such as different types of springs, could alternatively be used. Even combinations of mechanical and gas pressure controlled systems are fully conceivable within the fundamental concept of the present invention.
As indicated above parts of the gas pressure from the firing of the shell can be utilised to deploy the fins. Access to this propellant gas pressure is enabled by allowing it to enter the base-bleed unit, i.e. the unobstructed central passage of the propellant chamber. When the shell exits the barrel from which it is fired there is thus also a pressure inside the propellant chamber of the base-bleed unit that is equivalent to the pressure in the barrel. When the shell leaves the barrel the pressure outside the shell rapidly drops to normal atmospheric pressure, while the pressure inside the propellant chamber drops much more slowly as the sole opening of significance (to achieve pressure equilibrium) is the gas outlet of the base-bleed unit. Thus it is between the time when the shell leaves the barrel and before the pressure inside the base-bleed unit has had time to reach equilibrium (with the ambient atmospheric pressure) that the overpressure available is used to deploy the fins.
A special variant of the present invention utilises a removable protective casing that protects and retains the fins in retracted mode until the shell has left the barrel after being fired. An elementary way of mechanically removing this protective casing also involves using the gas pressure in the barrel during firing and allowing it free access to the inside of the casing. When the shell reaches the muzzle a pressure equal to the pressure in the barrel also exists inside the protective cover, but as soon as the shell exits the muzzle the pressure outside the cover rapidly drops to the ambient atmospheric pressure while the pressure inside the protective cover falls more slowly, resulting in this internal overpressure ejecting the protective cover against the sole smaller resistance offered by the atmospheric pressure. As already described, the same internal overpressure can also be used to deploy the fins.
Radially retracted fins have, of course, existed previously, but as far as we are aware they have never been directly retracted into the propellant chamber of a base-bleed unit in the way described in the present invention, where the fins in retracted mode are also protected by radial support guide-walls that have the double function of acting as active propellant supports.
The present invention is defined in the subsequent patent claims and is now described in more detail with reference to the illustrations shown in the appended FIGS. 1-5.